This invention relates to microwave energy curing of material and more particularly to hardening gasket material for closures used, together with a jar, to ensure package and material integrity within the jar.
It is known in the art that perishable material, such as food, is preserved in jars by using a closure comprising a cap and cap gasket to enclose the perishables within the jar. More specifically, during processing, food is placed in a jar. The air within the jar is then evacuated. Next a cap, with a cap gasket, is screwed onto the jar. Caps are typically made from a hardened plastic such as polypropylene. Hardened plastic caps do not seal to glass jars well enough to prevent the outside air from re-entering into the jar. Accordingly, in some applications, a cap gasket material that does seal to the jar is implanted in a groove formed on the inside of the top of the cap. When such a cap is placed on the jar, the cap gasket material will mold to the shape of the jar.
One such cap gasket material is a combination of a vinyl chloride polymer such as PVC powder and a plasticizer that is a member of the phthalate family, such cap gasket material comes in a liquid form. The cap gasket material is first placed in the groove and is then cured by being heated to a 340.degree. F. temperature. After such heating, the cap is cooled to room temperature, more specifically a closure of such type is constructed by the following steps (more detail of this process is explained in U.S. Pat. No. 4,309,744): First, the liquid cap gasket material is injected into a groove on the inside of the top of the cap. Second, the cap and cap gasket are pre-heated in a conventional heating oven to 300.degree. F. Third, the cap gasket, which absorbs microwave energy, and cap are then placed in a 10 kilowatt multi-mode industrial microwave oven and heated until the cap gasket material reaches a curing temperature of 340.degree. F. The cap itself is essentially transparent to microwave energy. Thus, the cap itself remains relatively cool because it does not absorb microwave energy. After the cap gasket material reaches curing temperature by the absorption of microwave energy, the closure is then removed from the microwave oven and allowed to cool, at which time the cap gasket material will harden.
The purpose of the pre-heating step is to prevent temperature gradients from developing within the cap gasket material. That is, if the cap and cap gasket material were inserted into the multi-mode oven at room temperature, as the gasket material absorbed energy to reach 340.degree. F., the cap, being non-absorptive to the energy, would act as a heat sink for the energy absorbed by the gasket material. Thus, the portion of the gasket material on, or adjacent to, the cap would be cooler relative to the portion of the gasket material furthest from contact with the cap. Thus, a temperature gradient would be developed across the gasket material. Temperature gradients may cause uneven hardening in the cap gasket material. Further, if the temperature in the cap gasket material becomes too hot, it may scorch, which results in the cap gasket material turning brown. If the temperature in the cap gasket material does not reach 340.degree. F., the cap gasket material will not cure and accordingly will remain liquid. If the closure is heated too long, the cap may melt and become distorted.
It is desirable, however, to eliminate the pre-heat step. One drawback of preheating is that the preheat oven requires a large amount of floor space. Further, preheating is an additional process step. Another drawback of preheating is that caps side walls may have a thin cross-sectional area that will deform if pre-heated.
Another drawback in using a multi-mode oven is that because it may not always be known how many closures are in process at any given time and because as closures absorb a percentage of the surrounding microwave energy, the requisite processing time may vary from load size to load size. This variation complicates the microwave oven processing procedure by requiring heating parameters to be constantly changed with load variations.
When processing many closures simultaneously, a large amount of floor space may be required to accomplish preheating. A multi-mode microwave oven may require 100-200 square feet of floor space. However, in a production environment, available floor space is typically very limited.
Finally, continuously processing closures having a height greater than 11/2" in a multi-mode microwave oven may not readily be accomplished without requiring elaborate choking structures to reduce leakage of microwave energy.